1. Field of the Invention
The present invention relates to a resistance calibration method and a calibration system, and more particularly, to a resistance calibration method and a calibration system capable of reducing the usage of external reference resistors and dummy pads.
2. Description of the Prior Art
On-die termination (ODT) is the technology where the termination resistor for impedance matching in transmission lines is located inside a semiconductor chip instead of on a printed circuit board (PCB). In order for a transmission line to minimize distortion of the signal, the impedance of every location on the transmission line should be uniform throughout its length. If there is any place in the line where the impedance is not uniform for some reason (e.g., open circuit, impedance discontinuity, different materials), the signal gets modified by reflection at the impedance change point which results in distortion, ringing and so forth. In order to avoid impedance mismatch, termination impedance with the equivalent amount of impedance should be placed at the end point of the transmission line. This is described as “termination”. There are several ways of termination depending on how the resistors are connected to the transmission line. Parallel termination and series termination are examples of termination methodologies.
Instead of having the necessary resistive termination located on the motherboard, the termination located inside the semiconductor chips is called ODT. In order to achieve better signal integrity and higher operational speed of data buses, the accurate ODT values are necessary. The conventional ODT calibration implementation is accomplished with an external reference resistor, which is connected to the chip via a dummy pad. A resistor coupled to the dummy pad is calibrated with the external resistor, and then the resistance setting of the dummy pad is duplicated to other pins on the chip, to realize the ODT calibration on all pins.
However, according to the conventional ODT calibration method, the external reference resistor and the corresponding dummy pad is always required for each chip on the PCB. Thus, there is a need to provide another resistance calibration method, in order to reduce the usage of external reference resistors and decrease the pin number of the chip, and thereby save the cost and area on the PCB.